Such adjusting devices are, for example, used for activating valve flaps. They usually comprise a hydraulically acting cylinder, which is arranged in a housing, the piston rod emerging through an opening in one end of the housing. With linearly moving valve flaps, the linear movement of the piston rod is usually transferred directly. With revolving valve flaps, the linear movement of the piston must initially be converted to a rotation. This, for example, takes place by means of a steep pitch between the piston rod and the housing, which causes a rotation of the cylinder rod. Another possibility is provided by the use of a toothed rod with a corresponding pinion, the conversion of the linear movement to a rotary movement then typically occurring outside the housing.
We distinguish between double-acting and single-acting adjusting devices. Double-acting adjusting devices are controlled via two pressure pipes, the valve flap opening when a control pressure is applied to a first pressure pipe and closing when a control pressure is applied to a second pressure pipe. Double-acting adjusting devices are usually dimensioned in accordance with the rotation torque and the rotation angle, or in accordance with the lift power and the lift length, respectively, which is required for a reliable activation of the valve flap.
For single-acting adjusting devices, however, one single control pressure pipe is sufficient. Here, the piston is moved by the control pressure against the force of a spring, which ensures the reset movement of the piston in case of a reduction of the control pressure. This has the advantage that also in connection with a defect, for example a pressure loss in the control pipe, a reset movement of the piston can take place. In connection with valve flaps, the opening often occurs by applying a control pressure, whereas the closing takes place by means of spring force. This ensures that the valve will also be closed in connection with a defect in the system. However, the valve can also be controlled in the opposite manner, so that it closes when a control pressure is applied.
The dimensioning of the spring takes place on the basis of the required closing force, both losses in the springs and the slackening of the spring force during reducing spring tension being considered. For example, cup springs or coil springs are used as springs, and they are suspended between one of the piston sides and the corresponding end of the housing. Also the use of various kinds of gas springs is known.
One disadvantage of single-acting adjusting devices with this embodiment is that the piston is constantly exposed to the spring force. Further to the adjusting force for the activation of the valve flap, an additional force must be provided to balance the spring force. This force could be provided by an increase of the control pressure, which is often only possible with great effort. Therefore, usually the effective piston surface is enlarged. With unchanged control pressure, the larger cross-section will also increase the total tension in the housing, meaning that the housing has to be dimensioned to be more robust.
This causes that single-acting adjusting devices require more space than double-acting adjusting devices with the same torque or the same lift force. This results in higher manufacturing and transport costs. Further, the larger dimensions are particularly disadvantageous in connection with the use onboard ships, as the additional weight makes a vibration safe mounting more difficult.
The invention is based on an adjusting device as known from DE 195 43 237 A1. Here, a hydraulic adjusting device is disclosed, which comprises a main cylinder and a separately located spring cylinder. The separate location causes that the tensioning of the spring does not occur via the main piston, so that the main cylinder can be made smaller than it is usually the case with single-acting adjusting devices.
The main piston is moved to the opening position in that a control pressure is applied on a first pressure chamber of the main cylinder. At the same time, the control pressure is applied on a pressure chamber of the spring cylinder, so that the spring is compressed. A reduction of the operating pressure, whether caused by a defect or by an intended intervention, causes a release of the spring. This presses pressure means from the pressure chamber of the spring cylinder into a second pressure chamber of the main cylinder. As the first pressure chamber of the main cylinder has at least a reduced pressure, the main piston is moved to the closing position. This means that a pressure drop causes a hydraulic transfer of the spring force to the main piston. Thus, a transmission of the spring force may occur, meaning, for example, that a shorter spring length can be used, which causes a more compact embodiment. The hydraulic transfer also reduces the oscillation inclination of the adjusting device, as the hydraulic fluid serves as damping.
EP 0 902 195 A1 shows an adjusting device, in which a spring cylinder is located outside the main cylinder. The spring cylinder is prestressed by the control pressure, which also activates the main cylinder. During a pressure drop, the spring force is mechanically transferred to the main piston, a transmission of the spring force not being provided.
In the known adjusting devices, the adjusting force, which is generated by the spring and is supposed to move the main piston back to the closing position, is reduced with increasing release of the spring.
If a valve flap is moved by the main piston, the valve flap must usually engage in the closing position to ensure a reliable fit of the closed valve flap. For this purpose, an additional force is required at the end of the closing movement. The spring is dimensioned in accordance with the force, which is required for a safe closing of the valve flap, which occurs at the end of the adjusting movement. This causes that over a large area the spring is overdimensioned, as the spring force is reduced with increasing release of the spring.
At the beginning of the reset movement a similarly large force is often required as for the safe holding at the end of the movement, as initially the static friction of the stillstanding valve flap has to be overcome. During the largest part of the closing movement, however, only a small force is required. Thus, in a large movement area, the known adjusting devices are overdimensioned, which causes an additional weight and further basically increases the risk of overloading the piston rod or a spindle of the valve flap.